The invention relates to a method of storing natural gas. More particularly, the invention relates to storage of natural gas at near ambient temperatures using a solid adsorbent, in particular, a carbon molecular sieve.
Natural gas, being substantially methane, has a low molecular weight and it occupies a large volume per unit weight compared to liquid fuels. Thus, reducing the volume of natural gas is necessary for most practical applications of natural gas as a fuel. Methods for doing this could include (1) storage at high pressure, (2) liquefaction and storage at low temperatures, and (3) adsorption on solids. Of these methods the last is of particular interest since compact storage of natural gas at moderate pressures may be achieved, while avoiding the cryogenic temperatures needed for liquid natural gas or the high pressures needed for storage as a gas in reasonable volumes.
Conventional activated carbons have been found to have reasonable capacity for natural gas but higher capacity is believed necessary for commercial success. Inorganic zeolites have also been tried, but found to have relatively low capacity for natural gas.
In U.S. Pat. No. 4,820,681 the present inventor has disclosed an improved carbon molecular sieve which has a high capacity for carbon dioxide and may be characterized as hydrophobic, requires no binders, and may be carbonized in a single step.
Ideally, the carbon molecular sieve would be a solid block completely filling a container so that no void spaces are present which waste volume in the container. To obtain access to the carbon and for practical reasons it will often be used in the form of solid particles which are packed into a container. In such situations, the closest approach to completely filling the container is desired. The present invention is directed to achieve a high packing density of particulate adsorbents.
In U.S. Pat. No. 4,495,900 zeolite adsorbent is compressed to form a solid mass for use in adsorbing methane gas. In a similar method, a Saran copolymer was formed into monolithic disks capable of filling a vessel and carbonized for use in the storage of natural gas (S. S. Barton et al. "The Development of Adsorbent Carbon for the Storage of Compressed Natural Gas," Ministry of Transportation and Communications, Government of Ontario, June, 1985).
Another method of increasing packing density combines particles ranging from 4 to greater than 325 mesh, as disclosed in U.S. Pat. No. 4,972,658. Two methods are described, one of which involves first adding large particles of a particular size range and then, without disturbing the orientation of the first particles, adding smaller particles having another defined size range to fill the spaces between the large particles. Such a procedure can yield non-reproducible results owing to the wide distribution of particle size and non-uniform shape. The present inventor has sought an improved method which can provide reproducible results.